1. Field of the Invention
This invention relates to a communication system including an interrogator and a responder (a transponder).
2. Description of the Prior Art
There are movable-object identification systems in which a responder (a transponder) mounted on a movable object can communicate with a fixed interrogator by radio. When the responder receives an interrogation signal (a challenge signal) from the interrogator, the responder automatically transmits a reply signal including identification information related to the movable object. The movable-object identification systems can be used as noncontact card systems in which a reader communicates with an information card by radio to read out information therefrom.
Responders in the movable-object identification systems are of two types having and lacking a data writing function respectively. The responder lacking the data writing function merely transmits internal data in response to an interrogation signal from an interrogator. The responder having the data writing function includes a device for decoding data transmitted from an interrogator, and a device for storing the decoded data.
Typical data transmission between an interrogator and a responder is of a serial communication format. According to such serial communication, data is modulated with a transmission clock signal before being transmitted from the interrogator. In the responder, the data transmitted from the interrogator is recovered by a demodulating process responsive to a reception clock signal. For accurate and reliable data transmission, it is necessary to equalize the periods of the transmission clock signal and the reception clock signal. When the period of the reception clock signal differs from the period of the transmission clock signal, the timing of the demodulating process goes out of order and the recovery of the data is unstabilized so that errors tend to occur in the communication.
A general way of generating the reception clock signal is as follows. The transmission speed in serial communication is preset. A reception clock signal is generated on the basis of the frequency of oscillation of an oscillator provided in the responder, and the period of the reception clock signal is accorded with the period of the transmission clock signal.
When the frequency of oscillation of the oscillator in the responder has an error, the period of the reception clock signal goes out of order so that a reception error tends to occur. Accordingly, the oscillator in the responder is required to be highly accurate in oscillation frequency.
It is known to use a crystal or ceramic oscillator as an oscillator in a responder. In this case, the oscillator tends to be expensive and large in size. Another type of the oscillator in the responder includes a CR (capacitor-resistor) oscillator and a circuit for stabilizing the frequency of oscillation of the CR oscillator. This type of the oscillator tends to be sensitive to a temperature variation. In addition, stabilizing the frequency of oscillation of the CR oscillator tends to be troublesome.